Laser leaves ash film on top

Hi I have the j-tech 7 watt laser that I have been trying to perfect photo lasering on different materials ceramic wood canvas Etc lately on canvas which is painted white and then black on top has been leaving a film sort of all I can figure is it’s Ash or some of the burn off I have to take the canvas and wash it off and if I’m not careful at all doesn’t wash off it will smudge
After I wash and before pictures this did not happen till recently I have tried adding air assist problem continues